Powdered ammonium nitrate explosive containing polyacrylamide and an inorganic cross-linking agent



POWDERED AMMONIUM NITRATE EXPLOSIVE CONTAINING POLYACRYLAMIDE AND AN IN- ORGANIC CROSS-LINKING AGENT Herbert L. Bowkley and John P. Merryweather, New

Castle, Pa., assignors to American Cyanamid Company, New York, N.Y., a corporation of Maine No Drawing. Filed Aug. 11, 1961, Ser. No. 130,773

8 Claims. (Cl. 14960) This invention relates to an improved, dry, explosive ammonium nitrate composition. More particularly, it is concerned with a composition which, on addition to water, is very rapidly converted to a soft, highly-viscous, but cohesive gel.

Ammonium nitrate explosive compositions in the form of aqueous gels have become the subject of a marked increase in commercial interest. For example, in British specification 825,367, published December 16, 1959, one type of such gel explosives is shown. In the copending application for United States Letters Patent Serial No. 130,774, filed of even date by Joseph A. Hoifman and Herbert L. Bowkley, one of the present inventors, further improvements are set forth. However, in field use certain conditions are encountered for which such compositions are not entirely suitable.

Typical explosive gel compositions of the above-noted application may have the following content ranges for their essential components:

Content Range (Wt. percent) Component: General Preferred Ammonium nitrate 25- 6O 30- 40 Metal nitrate 25 22- 24 Sensitizer 15- 50 28- 32 Water 5- 25 8- 15 Polymer 0.1-5 .0 1- 2 Metal salt 0.01-O.5 0.1-0.2

It will be noted that they contain an appreciable water content, usually some 5-25 weight percent of the total composition. Accordingly, shipping over appreciable distances involves undesirable haulage charges for this water content. In other frequently-encountered situations, it is desirable to add the explosive composition to water-filled boreholes or to place them under water. Handling of a gelled composition under such circumstances is not convenient. In many cases they cannot be efficiently used.

It would seem an obvious solution to both problems merely to omit the water content, ship the combined dry ingredients and add the water at the point of use. Unfortunately, this obvious-seeming solution appears to be more simple than is found to be the case when it is attempted. It proves on trial to be impractical, if not substantially impossible.

For example, an essential characteristic of a composition of the present invention is that it be thoroughly water-permeable and capable of substantially immediate and uniform wetting throughout the mass. Simply omitting the water content from a composition of the abovenoted copending application will not produce the desired result. Such a mixture, after being Wetted in accordance with the present invention is quite nonhomogeneous and for from being comparable, for example, with those disclosed and claimed in the present or the above noted copending application.

F 3 ,0 9 7,121 1C6 Patented July 9, 1 963 A principal advantage of the present invention is that it provides a suitable dry composition which can be stored almost indefinitely without deterioration. Thereafter, when combined at the point of use with the requisite amount of water a gelled composition having the desired explosive properties forms very rapidly, in some cases almost instantaneously. For example, a dry powder composition of this invention may be poured directly into a water-filled borehole by use of a pouring tube which is withdrawn as the explosive composition fills the hole. A satisfactory, useful gel is quickly constituted. The dry powder composition may even be placed in position under water, without being confined as in a borehole. It will still constitute the desired cohesive explosive gel without being leached of its more soluble components.

In general, compositions prepared according to the present invention will contain certain characterizing ingredients. These ingredients and the range of content over which each may be varied is shown in the following tabulation.

Content range Component: (wt. percent-dry basis) Ammonium nitrate (AN) 25-50 Metal nitrate 5- 25 Sensitizer 20- 40 Gel-forming agent 0.5- 5 Catalyst salt 0.05-0.5 Liquid (water) 0.6-2.5

In addition to the obviously expected decreased water content, the liquid used is for ease in blending only. It need not be water but may be any other inert, neutral liquid. It is preferably one which is water-soluble, i.e., alcohol, glycerine, ethylene glycol and the like. Petroleum oils and the like, though not water-soluble, may be used.

Preparation of a suitable composition within the scope of the present invention cannot ordinarily be accomplished simply by selecting at random some value for each ingredient that will fall within the indicated ranges. For optimum results it must be recognized that there is an inter-relation of the optimum proportions of the various ingredients which correspond to a particular content of any single ingredient. In some instances, also, certain criteria must be observed in selecting not only the amount of any one particular ingredient but also as to its physical characteristics. Accordingly, the ingredients will be individually discussed. However, consideration of the individual ingredients in the following discussion does not coincide with the order of presentation of the foregoing tabulation.

It will be noted that the instant compositions in the ANzmetal nitrate weight ratio range is from about 1:1 to about 10:1. In the gel compositions noted above, this ratio ranges about from 1:1 to about 12:1. The ranges do overlap and this distinction is not a critical one. The same is true of the differences between AN:sensitizer weight ratio range of from about 5:2 to about 5:8 as compared with that of the gel compositions of from about 2.5:1 to about 4:1. The ANzgel-forming material ratio also is not critically different.

Critical distinctions of the present invention over that of the above-identified copending application are found however in several respects. One is in the nature of the gelling agent used. Another is in the method of producing the final composition.

As in the gelled compositions, only p olyacrylamide of certain specific limitations appears to be a satisfactory gel-former. One critical distinction, however, is that the preferred type of polyacrylamide gelling agent suitable for use in the present invention dilfers from that for making preformed gel compositions. For the latter purposes, ahigh apparent molecular weight is important in obtaining optimum results. Polyacrylarnides for preformed gel compositions preferably should have an apparent'molecular Weight from Well above five million, up to some eight to fifteen million or higher.

In dry compositions of the present inventionpolyacryh amides having an apparent molecular weight above about 3.5 million are not generally useful. Polyacrylamides having a minimum apparent average molecular weight of about 100,000, i.e., containing at least about 2,000' acrylamide units are necessary, For optimum results an apparent molecular weight of from about one up to about three million is preferred. As high as five million is definitely too large for the preferred proportions.

A further essential characteristic of optimum composiing or dusting does not interfere with use of the coarse prills for the present purposes.

In some cases it may be desirable to include as a part of the AN content, AN particles which are considerably smaller in particle size. These will be referred to herein for simplicity as AN-fines. In general, where these AN-fines are present, they will be in the particle size range from about mesh to about +100 mesh on the Us. standard screen size table. These fines may be present from as little as the amounts normally found in the commercial grade of prilled product up to substantially the total weight of the AN.

One or more metal nitrates usually are found as characteristic ingredients in such compositions. In most cases it is sodium nitrate. These were originally incorporated to act as oxygen carriers. In the present invention they have an additional valuable function in that they seem to impart addition-a1 strength to the final gelled composi- IiOIl.

As seen from the foregoing tabulation, the general range over which the content of these nitrates may vary is itself fairly wide. :In general, an amount anywhere within this range may be employed. These should be at tions according to the present invention is that when wetted, they must be capable of being very rapidly, if

' possible almost substantially instantaneously, cross-linked to form the gelstmcture. For this purpose an ionic cross-linkage, such as produced by using a polyvalent metal salt as a cross-linking agent is to be preferred over the covalent or hydrogen bond-type of cross-linkage formation.

'I -he composition is not particularly critical as to the nature o is rql va m me al sa tu' Sulfate works well and ordinarily wil l bejthe salt most commonly encountered. It willjbe usedias illustrative in this discussion. Salts o f othenmetalssuch as chromium, iron, tin and the like also maybeused. .If so desired, other lents.

acidic anions may be employedsuch as nitrates, chlorides and the like also may be used. p I

.Perhaps less critical, but nevertheless a definite distinction between compositions of the presentinvention and pre -gelled compositions is that ahigher amount of metal salt cross-linking agent is required. In typical pregelled compositions, the polyacrylamidegagent weight ratio may be as low as about 500: 1. In the present invention, this ratio will range upward from a minimum of about 100:1, a differentorderof magnitude.

Perhaps the next most important characteristic ingredient is the AN. In fact, it was the original purpose in producing aqueous explosive compositions to utilize an ingredient such as AN. Ordinarily, therefiore, the AN content will be as high as is consistent with obtaining the desired explosive characteristics. 7

In the foregoing tabulation, the AN content is indicated as ranging from about 25 to about 50 weight percent of the composition. In general, however, it will be found that in the usual practice the maximum amount commensurate with the desired physical and explosive properties is preferred.

It is an advantage of the present invention that the AN used need not be highly pure, nor in general is it limited to use of AN in anypar-ticular particle sizerange.

Fertilizer grade AN is entirely satisfactory. Usually.

such material is prilled to insure, so far "as possible, that thematerial is and will remain in'a fiowable dry state. Usually such prilled products range in particle size from about 10 to about 20 mesh for themajor portion, there being somewhat larger particles usually present and usually also a somewhat l-arger'amount of somewhat finer particles. An entirely satisfactory explosive composition according to the present invention may be made when the AN content is comprised of such a commerciallyavailable mixture. 'The small amount of keiselguhr or the like which is usually added during prilling to minimize pick-up of moisture from the air and to resist cakleast an amount in excess of that which will dissolve in the expected water content of the composition after gellation. In most cases, however, a range of some 2025% will be rfiound to constitute a good, average practice.

A further advantage of the present invention is that it is not necessarily limited to any one particular sensitizer. These include such materials as smokeless powder, trinitrotoluene (TNT) and other known nitrated equiva- These are available in various well-known commercial forms, both as such and in various admixtures such as the group of compositions commonly referred to as tol s. As shown in the tabulation above, the sensitizer content may be present inv amounts from as low as about 20' up to as high as about 40 weight percent. Compositions containing a smokeless powder typeof sensitizer will usually require somewhat larger amounts than are usually necessary than when using TNT or equivalents thereof. In some cases this may require the full indicated 40%, or more, up to as much as 45% in some cases. In general, other types than smokeless powders seldom will be used in excess of about 35 i The amount used is atfected not only by the particular sensitizer to be used, but to a considerable extent by the particle size. By proper choice of the sensitizer, the material in the dry state can be made explosively sensitive. For example, when using smokeless powder of large size, i.e., above about 20 mesh, a minimum amount of about 25% will be required. Somewhat less TNT will be successful under otherwise similar conditions.

Many dilferent, materials are physically capable of forming typical gels of one type or another. Those which appear to act in aqueous explosive compositions only to increase the apparent viscosity are found of little or no value. The resultant gel after wetting of the dry composition must be capable of retaining the various ingredients in suspension in properly spaced relationship to each other. The final gel composition also must remain sufficiently fluid, i.e., it must not be so excessively viscous that it cannot be caused to flow into complete contact with surrounding medium.

As is also well known, polyacrylamides may be bydrolyzed to introduced so-called free acid or carboxy groups. For the purposes of the present invention, it is necessary that there be present a satisfactory content of such free acid groupings. As a minimum, they should be present to about 0.1%. As a maximum, is found to be excessive. About 8% may be taken as a practical opera-ting maximum. In good average practice it will be found that from about one to about two percent ordinarily constitutes the preferred range.

As was noted above, the present invention contemplates a particular method by which the various ingredients are combined to obtain the final composition. This is a critical distinction. In ordinary methods of dry mixing the order of addition in general is not important. This situation does not apply here. They result, not in the essential uniform admixture, but in segregation of the ingredients according to their different natures and particle sizes.

In carrying out a mixing schedule in accordance with the present invention, the metal salt cross-linking agent is the ingredient to which the remaining ingredients are subsequently added. This is the reverse of the method of forming pro-gelled compositions in which the gel-form ing agent is always the last ingredient to be added.

The polyvalent metal, salt, illustratively aluminum sulfate, in a suitable mixing apparatus, is first dissolved in a small amount of liquid, usually water. A certain amount of liquid is required to prevent segregation of ingredients which otherwise results. In general, it should constitute from about 0.6 to not more than about 2.5 weight percent of the final mixture. Use of more than about 2.5% results, not in a satisfactory dry composition, but one which is sticky and of poor handling characteristics.

To the mixer is then added the requisite amount of the selected sensitizing agent and of the selected metal nitrate. Mixing is continued only until a uniform prodnot is obtained. Concomitantly, in a separate mixer, the requisite amount of ammonium nitrate and polyacrylamide is dry blended to a uniform mixture. This latter mixture is then added to that in the first mixer and the two are blended for a short period, usually only a few minutes. This blending period must be minimized. Although the time element per se is not critical, if blending is continued beyond the period required to obtain a substantially uniform admixture, the action begins to resegrega-te the products. As soon as substantially uniform admixing is obtained, the product is ready to be dumped and packed for storage or use.

In this mixing procedure, it is not essential that the polyvalent metal salt be dissolved. It is, however, essential that there be sufficient amount of liquid present to prevent particle segregation in the first mixer. For this purpose other liquids such as glycerine or glycol may be used as noted above. It is only necesary that the polyvalent metal salt be uniformly distributed through the liquid mass before additional solids are added. This is done to more readily insure uniform distribution of the metal salt through the pro-mixed mass.

The final mixed product should be stored under conditions which prevent any unnecessary contact with -atmos pheric moisture. it has been found that impermeable plastic bags are highly suitable. This also applies to boxes having a lining of the impermeable plastic. For this purpose polyethylene or other polymeric plastics capable of forming an impervious strong sheet or film have proved entirely satisfactory.

The invention will be more fully illustrated in conjunction with the following examples which are intended as illustrative only. Therein all parts and percentages are expressed by weight unless otherwise stated.

Example 1 A 60 lb. dry-mix composition was prepared according to the procedure outlined above. The ammonium nitrate contained a small proportion of fine sizes. Ethylene glycol was used as the liquid. The following formula- 6 tion shows the actual dry product composition and the approximate composition after adding water with resultant gel formation.

Content (weight percent) Component Dry Wetted AN +20 mesh 33.06 29. 26 AN 20 +100 mesh- 4. 54 4. 02 24. 21.68 32.80 29.03 2. 71 2. 40 2.17 1. 92 0.22 0. 19 2 ll. 50

1 85% nitrocellulose, 10% dinitrocellulose and 5% plasticizer and stab izer.

2 Not a limiting factor.

The resultant soft pourable spongy mix was packed into fiberboard cases provided with a polyethylene liner. For convenience only, no top was placed on the box, the liner being closed by tying off with cord. The sopackaged mixture was placed in magazine storage. Periodically the package was examined and samples withdrawn. in each instance the product was found to continue to be as soft and spongy as when first packed.

The periodically withdrawn samples were converted to gel form by addition of sufficient water to constitute gels containing variously from about ten to about thirteen weight percent of water. Content of the gel at an average water content of about 11.5 weight percent is indicated above. Gelled samples. were exploded unconfined using a g. pentolite primer. The critical diameter was below four inches.

Example 2 Another dry-mix composition was prepared according to the procedure of Example 1 but substituting TNT for the smokeless powder. The mix proved equally storage stable and readily constituted to gel form. Detonation of so-constituted gels was obtained using a 20 g. pentolite primer.

Example 3 In order to illustrate the effect of varying the sensitizer particle size, the procedure of Example 1 was repeated using smokeless powder ground to varying particle size ranges to obtain a number of samples. These samples were gelled by addition of sufficient water to produce a water content of fifteen percent. Resultant gels were detonated using the minimum effective amount of primer (pentolite) and the critical diameter measured. illustrative results are shown below.

Screen Size 1 Minimum Critical primer 5 diameter 3 l U.S. Standard. 2 Grams. 3 Inches.

It is clearly seen that the sensitivity increases with decreasing particle size. Accordingly, when the sensitizer is largely coarse grained, i.e., above 20-30 mesh, a minimum of about 25% should be used, particularly in the case of smokeless powder, a good average practice being to employ from about 28 to about 32% by weight of the whole composition. \In the case of TNT, these limits usually may be smaller by some three to ten percentof the total composition. Hence, the lower limits as-to sensitizer content and the higher AN2-sensitizer ratios in the ranges discussed above.

In the foregoing examples, TNT and smokeless powder storage for 30 days.

in commercial grades were used. Perhaps a more desirable but not essential variation is obtained if a sensitizer composition is separately blended and used for the 1 Oyclotrimethylenetrinltramine.

Such compositions are used to replace the sensitizer in the compositions noted in the discussions above.

Example 4 Another dry mix was prepared as in Example 1. It differed in that all the AN was in the form of coarse grains; a composite sensitizer having the formulation immediately above was substituted for the smokeless powder, and water was used as the mixing liquid. Resultant dry mix has a density of about 1.1 gm./cm. in

V the following proportions:

Content (weight percent) Component Dry Gelled AN (+20 mesh) 38. 60 32.17 NaNOQ 25. 20 21.00 Sensitizer 33. 80 28. 17 Polyacrylamide. 1. 53 1. 27 AMSOQ; 0.15 0.12 Water 0. 72 17. 27

About 50 lbs. was stored in a'large polyethylene bag in an open top fiberboard box, the bag being tied off with string. Substantially no change occurred in magazine It was gelled by adding the water directly'into the top of the bag. Penetration of water was complete and uniform in about fifteen minutes to obtain complete gelation. The gelled product was stored in the same bag. Periodically samples were removed, examined and then detonated in four-inch polyethylene bags. No apparent change occurred in storage. Detonation was successful using a 20 g. pentolite primer.

About half the remainder of the dry mix was added to the water in a four-inch diameter polyethylene tube. Water penetration was complete and uniform. Detonation was obtained using a 20 g. primer.

The remaining dry mix was poured into a large excess of water. Gelation was complete and rapid. Detonation was obtained using a 60 g. primer.

Example Example 3 was repeated using the formulation of Example 4 to obtain a number of samples in which the composite sensitizer was present in varying particle size. On addition of the water to the dry powder in a polyethylene bag, gelation was rapid, complete and uniform in each case. Using a primer (pentolite) the critical diameter was determined. Illustrative results are shown Such compositions, like smokeless powder, have the advantage-of increasing sensitivity as the average particle size decreases. In this respect they differ from sensitizers like TNT which have optimum eflicacy between about 10 and 16 mesh and decrease 'in sensitivity as the average particle size is decreased below that nange. Where increased sensitivity is important, this is an advantage of the composite sensitizers in that they provide for an added control in the ultimate product.

We claim:

1. A stable, dry, pourable powder composition adapted to form on contact with water a soft, non-separating, highly-viscous, cohesive explosive gel, said powder composition containing as its essential ingredients (a) from about 25 to about 50 weight percent of ammonium nitrate; (b) from about 5 to about 25 Weight percent of an alkali-metal nitrate; (c) from about 20 to about 40 Weight percent of a nitrated explosive sensitizer; (d) from about 0.5 to about 5 weight percent of a polyacrylamide having an apparent molecular weight of at least 100,000; (e) from about 0.05 to about 0.5 weight percent of a polyvalent metal inorganic salt cross-linking agent for said polyacrylamide; and (f) from about 0.6 to about 2.5 weight percent of an inert liquid 2. A powder composition according to claim 1 in which said polyacrylamide contains at least 2000 acrylamide units and an apparent molecular weight of from about 100,000 to about 3.5 million.

3. A composition according to claim 2 in which said polyacrylamide has an apparent molecular weight of from about one to about three million.

4. A composition according to claim 1 in which the weight ratio of ammonium nitrate to alkali-metal nitrate is from about 1:1 to about 12 21.

5. A composition according to claim 1 in which the weight ratio of ammonium nitrate to explosive sensitizer is from about 5:2 to about 5:8.

6. A powder composition according to claim 1 in which the weight ratio of polyacrylamide to .polyvalent metal salt is from about 10:1 to about 15:1.

' weight percent of the dry powder composition'according to the steps which comprise: adding to an operating mixer from about 0.05 to about 0.5% of an inorganic, watersoluble salt of a polyvalent metal, about 0.6 to about 2.5% of an inert liquid; adding thereto from about 20 to about 40% of a nitrated explosive sensitizer and from about 5 to about 25% of an alkali-metal nitrate; stopping the mixing as soon as a uniform blending is obtained; forming a dry-blended mixture of from about 25 to about 50% of ammonium nitrate and from about 0.5 to about 5% 'of a polyacrylamide, said polyacrylamide containing at least 2000 acrylamide units and an apparent molecular weight of at least 100,000; adding said dry-blended mixture to said mixer; starting said mixer and continuing resultant blending until a uniform admixture is obtained and stopping said mixing before resegregation of the ingredients is initiated.

References Cited in the file of this patent UNITED STATES PATENTS 3,000,175 Lawrence Sept. 19, 1961 

1. A STABLE, DRY, POURABLE POWDER COMPOSITION ADAPTED TO FORM ON CONTACT WITH WATER A SOFT, NON-SEPARATING, HIGHLY-VISCOUS, COHESIVE EXPLOSIVE GEL, SAID POWDER COMPOSITON CONTAINING AS ITS ESSENTIAL INGREDIENTS (A) FROM ABOUT 25 TO ABOUT 50 WEIGHT PERCENT OF AMMONIUM NITRATE; (B) FROM ABOUT 5 TO ABOUT 25 WEIGHT PERCENT OF AN ALKALI-METAL NITRATE; (C) FROM ABOUT 20 TO ABOUT 40 WEIGHT PERCENT OF A NITRATED EXPLOSIVE SENSITIZER (D) FROM ABOUT 0.5 TO ABOUT 5 WEIGHT PERCENT OF A POLYACRYLAMIDE HAVING AN APPARENT MOLECULAR WEIGHT TO AT LEAST 100,000; (E) FROM ABOUT 0.05 TO ABOUT 0.5 WEIGHT PERCENT OF A POLYVALENT METAL INORGANIC SALT CROSS-LINKING AGENT FOR SAID POLYACRYLAMIDE; AND (F) FROM ABOUT 0.6 TO ABOUT 2.5 WEIGHT PERCENT OF AN INERT LIQUID.
 8. A METHOD OF PREPARING A STABLE, DRY, POURABLE POWDER COMPOSITION ADAPTED TO FORM ON CONTACT WITH WATER A SOFT, NON-SEPARATING, HIGHLY-VISCOUS, COHESIVE EXPLOSIVE GEL WHICH COMPRISES COMBINING AMOUNTS EXPRESSED IN WEIGHT PERCENT OF THE DRY POWDER COMPOSITION ACCORDING TO THE STEPS WHICH COMPRISE: ADDING TO AN OPERATING MIXER FROM ABOUT 0.05 TO ABOUT 0.5% OF AN ALKALI-METAL NITRATE; STOPPIN SOLUBLE SALT OF A POLYVALENT METAL, ABOUT 0.6 TO ABOUT 2.5% OF AN INERT LIQUID; ADDING THERETO FROM ABOUT 20 WEIGHT PERCENT OF THE DRY POWDER COMPOSITION ACCORDING ABOUT 5 TO ABOUT 25% OF AN ALKALI-METAL NITRITE; STOPPING THE MIXING AS SOON AS A UNIFORM BLENDING IS ABOUT; FORMING A DRY-BLENDED MIXTURE OF FROM ABOUT 25 TO ABOUT 50% OF AMMONIUM NITRATE AND FROM ABOUT 0.5 TO ABOUT 5% OF A POLYACRYLAMIDE, SAID POLYACRYLAMIDE CONTAINING AT LEAST 2000 ACRYLAMIDE UNITS AND AN APPARENT MOLECULAR WEIGHT OF AT LEAST 100,000; ADDING SAID DRY-BLENDED MIXTURE TO SAID MIXER; STARTING SAID MIXER AND CONTAINING RESULTANT BLENDING UNTIL A UNIFORM ADMIXUTE IS OBTAINED AND STOPPING SAID MIXING BEFORE RESEGREGATION OF THE INGREDIENTS IS INITATED. 